Wireless communication systems implement a variety of radio frequency (RF) power amplifier topologies. The current trend towards highly linear multi-carrier RF transmitters and microwave transmitters having high power efficiency requires more sophisticated circuit topologies, such as class F amplifiers, Doherty amplifiers, and various switch mode RF power amplifier families. Due to the rigorous noise requirements of the new generation wireless systems, some linear power amplifier, such as classes A, AB and B, are forced to operate at low power efficiency, which increase the operating costs of the wireless network. Since power amplifiers consume about 60% of the total power in a transmitter system, highly efficient topologies are need.
With semiconductor technologies pushing clock speeds, switch mode power amplifier (SMPA) systems are becoming an attractive choice over the conventional linear power amplifiers by offering higher efficiency, reduced size, and lower cost. Among switch mode power amplifier topologies, the Class S power amplifier, which is widely used at low frequency, becomes a promising configuration suitable for wireless systems operating at high frequency, such as the new generation base stations using W-CDMA, UMTS, or CDMA2000 air interfaces. Working at a switching frequency higher than the signal frequency, the Class S power amplifier offers a lower harmonic distortion than an amplifier that switches at the signal frequency.
However, the number of available applications of Class S power amplifiers is limited. Many of these applications are directed towards the use of band-pass delta-sigma modulators for either baseband audio applications or intermediate frequency (IF) applications with much lower sample rates than those being considered for the UMTS band and other wireless communication bands. Commercial applications of the band-pass delta-sigma modulator (BP-DSM) are mainly for GSM at intermediate frequency (i.e., IF=70 MHz) and relatively low frequency RF carriers (i.e., RF=250 MHz).
However, many commonly deployed wireless systems operating at 1.9 to 2.1 GHz require a high sampling rate and switching rate of about 8 GHz. To implement a BP-DSM at 8 GHz forces the design out of inexpensive semiconductor processes for implementation. A delta-sigma modulator (DSM) operating at an 8 GHz switching frequency is also more susceptible to parasitic effects that could degrade the loop performance or cause instability. Also, operating a switch mode PA at 8 GHz requires a power device having a high fT. In practical terms, power devices with higher fT have lower peak operating power. For these reasons, the significant limiting factors in implementing a Class S power amplifier in the 1.9 to 2.1 GHz frequency bands are the cost and performance limitations of implementing circuits with switching frequencies in the 8 GHz range.
Therefore, there is a need in the art for an improved radio frequency (RF) power amplifier for use in wireless communication systems. In particular, there is a need for an improved RF power amplifier for amplifying RF signals with high power-added efficiency.